There are many names commonly used interchangeably for the monofilament line used for cutting vegetation, including not limited to the following: trimmer line, weed trimmer line, grass trimmer line, monofilament line, string trimmer line, strimmer line, cutting line, line, line strips, strips, flails, and weed whacker line. Monofilament line is sold in many different cross-sectional shapes and is made from many different types of nylon plastic. Some of the nylon plastics are more easily deformed than others and some materials have lower melting points than others.
The prior art, including U.S. Pat. Nos. 7,603,782 and 7,743,511 (hereby included by reference for their teachings), discloses trimmer heads with line holding mechanisms (line holders) capable of pivoting on a vertical axis. Names commonly used for these line holders includes but is not limited to: posts, pivoting line holders, pivoting posts, pivot posts, line holding mechanisms and pivoting line holding mechanisms. All of these terms many be used interchangeably. These terms are used with line holders designed to pivot about a vertical axis. Some of these terms, however, are also used with line holders that are not designed to pivot.
The prior art discloses various designs for line holders, including line holders having a series of at least two, spaced but adjacent, parallel holes extending from end-to-end completely through the line holder, with the material between the two holes serving as a central member about which a bent back, flexible trimming line is positionable with the central crook of the bent-back line being wrappable around the central member and with the rest of the line segments positionable in and extending completely through the openings with the distal ends of the flexible trimming line extending out of and past the line holders.
Certain commercialized products incorporating pivoting posts include line-holding mechanisms having the basic shape of a round post with a circular flange at the lower end. The top end of the post is beveled (slanted) on two opposite sides with two straight-through passageways for insertion of the trimmer line, where the passageways connect the two beveled portions. The flange at the bottom of the post is sandwiched between a lower cover and the upper portion of the head and is held such that it can rotate about a central vertical axis but cannot be moved in a vertical manner. The pivot posts on commercial trimmer heads are injection molded as a single piece of plastic and have two parallel straight through holes with a portion of plastic separating the two through holes. A representation of these prior art commercial pivot posts can be found as reference numeral 50 in FIG. 19 of the '511 patent.
To load trimmer line in the posts of the prior art trimmer head, two opposed ends of a folded strip of line are threaded through the two straight parallel holes from a first side and the central folded portion of line is pulled tightly against the portion of plastic separating the two holes. Commercial trimmer heads utilizing this type of pivoting line holding mechanism can have two, three or four pivoting posts and include the PivoTrim™ X2, PivoTrim™ Ugly™ Head, and the PivoTrim™ Classic and PivoTrim™ Pro models, respectively. These products are sold by Shakespeare Company, LLC.
During use, the trimmer head is rotated by a string trimmer or brush cutter machine. With pivoting post trimmer heads, the ends of each folded piece of trimmer line will extend out radially due to the centrifugal force created by the rotation of the trimmer head. The tips of the trimmer line are able to cut vegetation due to its rotational velocity. If the trimmer line impacts an immovable object or very dense vegetation, the line and post can pivot away from the point of impact, which reduces the stress on the trimmer line. An advantage touted for trimmer heads with pivoting line holders is that the trimmer line is less apt to break if it impacts a fence or other immovable object. There are some shortcomings associated with the commercial designs of the pivoting post heads, however.
Over time, the trimmer line becomes worn and must be replaced. At that point, the folded strips must be removed and new strips inserted. Consumers sometimes report needing to use a pair of needle-nose pliers to pinch the folded portion of the line to remove the remaining U-shaped portion of trimmer line from the post. If the consumer does not have a tool that can grip the line, then they must stop using the trimmer head until a tool can be obtained to remove the lines. This is frustrating to the consumer.
Additionally, over time the portion of plastic separating the two straight through holes can become worn at the two opposite ends. This can occur due to a slight sawing motion that occurs when certain lines are used in the pivoting posts. If the consumer selects a trimmer line that is not held tightly by the post, it is possible for one of the two ends of trimmer line to be gripped more than the other upon impact, pulling on one leg of the line and making it shift through the two passageways of the post. The line shifting acts like a saw on the portion of the post separating the two through holes. Over time, this sawing motion can create a recessed area where the line can be pulled further into the body of the pivot post, making removal of the line even more difficult. The portion of plastic separating the two through holes continues to wear more and more over time and the post must eventually be replaced.
The above discussed sawing motion can also cause frictional melting of either the trimmer line surface or the surface of the portion of the plastic pivot post separating the two through passageways. Once the motion of the trimmer head stops, the melted portion cools, and the plastic line and plastic pivot post fuse together. This can make removal of the used portion of the line very difficult, even when using needle nosed pliers to grip the line remnant.
Another observation is that when the strip of trimmer line is folded, the portion of line at the fold will flare or bulge outward and perpendicular to the plane containing the folded line. The amount of flare varies with the shape and design of the particular trimmer line used. During use, the ends of the trimmer line are pulled with each impact, pulling the folded portion of line more tightly against the portion of the post separating the two through passageways. Thus, the trimmer line becomes more deformed at the fold and the flare becomes accentuated. If the sawing motion discussed above is also taking place, then the deformation associated with the flaring can be pulled towards and wedged into one of the two through holes in the post.
Another complaint is that the two legs of the trimmer line break flush with a slanted surface of the pivot post, leaving the folded portion of line still in place in the post and the short legs of line still inside the two parallel through holes. These short pieces can be difficult to remove. An investigation into scenarios where this is most likely to occur led to identifying the trimming of volunteers (small spouting trees, one to three feet tall) as a trimming application where the line is most apt to break flush with the post. It is thought that the trimmer line wraps around the trunk of the volunteer, thus interfering with the ability of the line to pivot out of the way.